The class of cutting tools known as a shear use two opposed and cooperating cutting edges to apply cutting force to a workpiece. Shears and scissors have a wide variety of uses. Shears and scissors are used for cutting paper, fabric, sheet metal, and many other types of sheet material. Shears are also used in gardening for pruning trees, shrubs, and other plants. Perhaps the most common type of shears is the class of shears having two blades with handles, the blades being pivotally connected at their center for pivotable movement between open and closed positions. This class of shears includes scissors and, therefore, shall be referred to herein as a scissors-type shears.
With scissors-type shears, it is common practice to bias the blades to an open position by means of a spring. With spring-biased shears, the user applies cutting force by squeezing the handles of the shears together, causing the blades to close. When the user relieves pressure on the handle, the spring urges the blades to an open position. Thus, the user is not required to apply force to open the blades of the shears. Spring-biased shears typically include a lock mechanism to maintain the blades in a closed position when they are not in use. Locking the blades in a closed position helps prevent damage to the cutting edges of the shears. Additionally, securing the blades in the closed position reduces the risk of injury because the cutting edges are not exposed when the blades are closed.
Many types of locking mechanisms have been devised in the past to secure the blades of scissors-type shears in a closed position. A common type of locking mechanism used in spring-loaded shears is a pivoting latch. Typically, a latch element is pivotally attached to one handle. The latch element includes a notch that engages with a latch pin on the opposing handle. An exemplary pivoting of latch mechanism is shown in U.S. Pat. No. Des. 406,507. Another common type of locking mechanism is a simple loop or bight element attached to one handle that engages a notch in the opposing handle when the shears are in the closed position. This type of locking mechanism is shown in U.S. Pat. No. 5,063,671. The locking mechanisms described above are relatively simple and inexpensive to manufacture. However, these locking mechanisms require two-handed operation: one hand to apply force to hold the shears in a closed position, and one hand to engage the latch or bight element. Also, while consumers may expect these types of locking mechanisms on inexpensive tools, using these mechanisms on more expensive tools could negatively impact sales since consumers may desire a more elegant locking mechanism in higher-priced tools.
Sliding lock mechanisms are also known for locking shears in a closed position. Examples of shears with sliding lock mechanisms are shown in the patent to Wallace et al., U.S. Pat. No. 4,156,311 and LaBarre et al, U.S. Pat. No. 5,367,774. The patent to Wallace discloses a sliding latch that slides back and forth in a slot formed in one of the handles. The opposing handle has a locking stud. The sliding latch slides into and out of engagement with the locking stud to lock and unlock the shears, respectively. The patent to LaBarre discloses a sliding lock mechanism comprising a pin that passes through aligned slots in the handles of the shears. The pin slides within the slots between locked and unlocked positions. The sliding mechanisms exemplified by these patents achieve the desired goal of one-handed operation. However, the sliding mechanisms of the prior art have various limitations. For example, some sliding mechanisms of the prior art do not retain their position during use and tend to interfere with the operation of the shears. Also, many sliding mechanisms are characterized by relatively complex construction having numerous parts. In general, an increase in the number of parts equates to greater material cost. Further, increasing the number of parts usually makes the assembly of the shears more difficult, further increasing the cost of manufacturing the tool. Additional parts also mean more opportunities for wear or failure, reducing the reliability of the tool.
Accordingly, there is a need for a new locking mechanism that is capable of one-handed operation, is simple in construction, and can be inexpensively manufactured.